There are many systems and methods for discharging solids from a fluidized bed pressure vessel, gas phase fluidized bed pressure vessel, or gas phase fluidized bed polymerization vessel. However, the use of existing discharge systems and methods can result in higher loss of reactants from the discharge system than is desirable. Specifically, a significant amount of the gas or gas/liquid mixture may be lost because the void space within and around the particles is filled with a high-pressure gas mixture. The lost gas must then be either replaced, consuming additional raw materials, or recycled back into the system via compression, condensation with pumping, or a combination of these. In either scenario, raw materials are wasted and energy consumed.
One process that involves the discharge of a gas/solids mixture from a pressure vessel is the process for the manufacture of polyolefin resins, thereby involving the polymerization of olefin monomers in a fluidized bed reactor. An example of a process for the manufacture of polyolefin resins is disclosed in, for example, U.S. Pat. No. 4,003,712 (“the '712 patent”). As therein defined, a product is discharged from the reaction zone through a gas lock zone and the unreacted monomer that accompanies the resin is vented and recycled back to the reaction zone by compression. The product is then transferred to downstream equipment via a conventional dilute phase conveying system.
An alternative discharge system is described in, for example, U.S. Pat. No. 4,621,952 (“the '952 patent”). The '952 patent describes a gas lock zone system involving multiple settling vessels operating in series. The '952 patent describes that the gas mixture lost from the process could be significantly reduced by using the gas displacing ability of solids using two or more vessels with pressure equalization between each. As described, a valve between a nozzle on the fluidized bed pressure vessel and settling vessel is opened, and solids along with pressurized gas enter settling vessel. A second connection between the top of the settling vessel to a slightly lower pressure section of the reactor provides a flow path for the gas while solids settle out to essentially fill settling vessel. Both valves are then closed, leaving the settling vessel full of the solid particles, but with interstitial spaces between the particles filled with the gas mixture, and the settling vessel at full reactor pressure. The settling vessels of the discharge systems described in the '952 patent typically comprise a hemispherical top head, a straight section and a conical bottom section. The second connection is typically in this hemispherical head.
After the settling tank is isolated from the fluidized bed pressure vessel, a valve is then opened and solids are transferred to a transfer tank. As the solids flow into the transfer tank, pressure equalization also occurs between transfer tank and settling vessel. Upon completion, the pressure in transfer tank and settling vessel are at a moderate level. However, the transfer tank still contains a substantial amount of gas in the interstitial spaces between particles. Some practitioners then open a crosstie valve to allow the moderate pressure gas to transfer to an empty transfer tank in another series of tanks. Once the pressure in the transfer tank is relatively low, the product is transferred to other vessels for additional processing with only a modest pressurized gas transfer therein. The gas retained in the settling vessel is transferred back into the fluidized bed pressure vessel during the next fill cycle.
Additionally, U.S. Pat. Nos. 6,255,411 (“the '411 patent”) and 6,498,220 describe the crosstie concept mentioned above in detail and describe various improvements. The '411 patent also offers a faster cycle time.
Other background references include U.S. Pat. No. 6,472,483, EP 0 250 169A2, and WO 2006/079774.
Accordingly, there exists a need for an improved method to remove matter, such as, primarily solids, from a fluidized bed pressure vessel with maximum volumetric fill of the settling tank, which results in greater efficiency in the processing of the matter, while addressing safety concerns of dealing with a pressurized reactor system.